Operation of 800-B Pushbutton Tuning System

[Joseph W Strickland]

The description of the set-up for the presets used in the E H Scott 800-B receiver is detailed enough on how to set the position for each of the station presets. What is not discussed is how the unit actually tunes to the station.

I am wondering if when a given pushbutton is depressed momentarily, that it engages the necessary relays to switch to the required frequency band plus starts the tuning motor and the motor remains engaged until it reaches the preset. Or, does the user have to keep the pushbutton depressed until the radio reaches the station selected?

Also, if the radio is turned on and operating on a given station that is below the next station to be tuned in with the electric tuning system, does it automatically tune up in frequency? In a similar situation where the radio is tuned to a frequency above a given station, does it automatically tune in a downward direction? Or does the unit tune in the same direction of travel when last used and then reverse at the end of the dial and come back to the station with the motor operating in reverse until it reaches the desired station?

Joe

[Joe Miller]

On mine, you must hold the PB down until the tuner arrives at the preset location.

The subsequent selection (again holding down the PB) goes directly to the preset location.

Hope this helps,

Joe

[Joseph W Strickland]

Joe;

OK, thanks. I was wondering about it because normally you would expect to see some sort of latching relay or some sort of circuit that would hold a relay engaged until the preset contact was reached. It appears that the bakelite insulated strip is used to break a circuit and definitely kill voltage going to the motor windings. The description also mentions a clutch action using the spring loaded shaft of the motor which has a switch built into the back of the motor assembly. This is going to be interesting to watch when I can get the radio up and operational. I now have a 10-pin plug coming from Allied Radio which may be here by next week.

I will try to do a more detailed analysis when I manage to apply power and make a report.

Joe

[David C. Poland]

Joseph  .... Yes - You push and hold a station pushbutton until the motor stops.   

For those who have not worked with this complicated pushbutton control mechanism:

That big round housing on the rear of the receiver contains a round disk split in half that acts as switch to control disk mechanical rotation. Preset "pins"  mount along a slot on the housing  and contact one or the other half of the split disk, depending on the current position of that disk (and dial). One half of the the disk runs the AC motor one direction, say for clockwise rotation, and the other half, counter clockwise. The slide rule dial moves across as disk rotates. Each push button on the radio panel (or optional remote keypad) is wired to a "pin contact". To select a station preset, push and hold the preset button until the "pin" contact reaches the gap between the disk halves, the motor stops and you have reached the station. 

The 800-B I restored originally had only 3 buttons factory dedicated to FM, but today most of the stations desired will probably be FM - and spread all across the dial. The fun is deciding how to choose the order of "pin" locations along the two slots on the housing and, yet, avoid physical interference of the adjustable "pin contacts" for closely  spaced intermixed AM and FM stations. And then rewire the AM or FM  push button harness for the desired push buttons for AM or FM stations - thus controlling the AM/FM relay  for respective AM or FM stations. Using a an AM or FM push button is the only means to change between an AM or FM band. (You may need to reread this paragraph a couple times)

Using my printer I made color coded call sign tabs on regular paper to cut up to insert into the pushbuttons -  brown background for AM call letters and green background for FM call letters. I choose a light yellow for the call sign letters for contrast. Took a couple drafts to get the right size font, color and contrast for the tabs. Think carefully what AM and FM stations you choose, as can be a real pain to change your mind a few days/weeks later. 

[Joseph W Strickland]

Dave;

Thanks for the additional confirmation on how the pushbutton tuning system works. I finally realized after close look at the schematic as I did a composite schematic showing figures 26 and 30 that there was no "latching" action to keep power applied and that the button had to be held down. To do it any other way would have required some sort of logic circuit whether done with relays or diodes and amplifier tubes etc. Philco had their "mystery remote" which they advertised as not using radio waves, but in fact it actually did. It used radio frequencies below the AM band.

We have about four AM stations that we listen to and perhaps three or four that we listen to on FM, so some of the contacts will probably never be used in our case.

I sent a *.png version of the composite radio-tuner and preset tuning schematic to our site here yesterday. I also sent a copy of a *.jpg version to Kent. I have a *.pdf version which is larger but I would need to deposit it on my DropBox site and send Kent a link for that version. I may be able to get that done later today.

Joe

[Norman S Braithwaite]

The push buttons on RCA Berkshire worked the same way. One had to hold the button down until the station was reached. Unlike the Scott, if you released the button before the station was reached, it was possible to confuse the electromechanical automatic tuning mechanism requiring some fiddling to restore operation. Going from recollection Chuck Dachis modified his Berkshire to include a mechanical push button latch released by a solenoid. I'm not sure this can be accomplished on the Scott without a major effort. Come to think of it, the Berkshire modification was probably a major effort.

Norman

[Joseph W Strickland]

Norman;

Thanks for that information on the RCA Berkshire electric tuning. I need to check with my friend who has one to find out if his Berkshire has this modification. Knowing Jake, I suspect that it does. He always kept up with the latest service information on RCA equipment, and especially for his equipment at home. I need to go visit him. He is now 92 years old and still a storehouse of information. My thought would be to find a way to use a latching relay that could be un-latched when a given station is reached. A mechanical latch for a pushbutton would be a real challenge to construct and would be significantly different from the original design.

Joe